A wide variety of electronic systems call for the capacity of producing a PWM signal, such as, in particular, in DC/AC converters, PWM inverters for driving alternating current motors, in control systems comprising phase locked loops (PLL), and similar systems.
A widely used modulating technique is the so-called sine-triangle technique. This includes comparing a first alternating signal which might be a sinusoid (customarily referred to as the modulating signal or the control signal) with a second alternating signal, essentially of a triangular shape (customarily referred to as the carrier signal or the reference signal).
In many applications, such as those cited above, variation of the frequency of the produced modulated signal takes place by varying the frequency of the control signal (for instance sinusoidal), whereas the variation of the generated voltage may be obtained by varying the amplitude of the triangular signal. In this type of system realized in a fully digital mode, the voltage and frequency variation is actuated via dedicated algorithms for the generation of digital data streams symbolizing the alternating control signals, by employing a microprocessor, often using a pre-established scheme in the form of a look-up table. In these systems, the number of values selectable or digital data are intrinsically limited in terms of available memory space. Furthermore, the control chain includes one or more microprocessors whose cost may have a substantial impact on the overall cost of these products.
Moreover, in recent years the development of DC/AC converters has made possible extending the use of the asynchronous motor in many industrial sectors. Prior to the evolution of converters, and when such energy conversion was not readily attainable, the motor could be rigorously used in line with nominal specifications (frequency and voltage) as defined by the manufacturer. Indeed, to vary speed and torque it is indispensable, to vary both the supply voltage and the frequency. Such a motor could be controlled only by the use of dynamic converters or costly mechanical transmission systems. Nowadays this machine is attracting a renewed interest because of its intrinsic simplicity and sturdiness that make it preferable to DC machines, in part, due to cost-effective new electronic technologies capable of providing reliable and compact switching devices.
To generate proper driving signals of variable voltage and frequency for an asynchronous motor modulation techniques are necessary, such as the sine-triangle PWM technique.
Obviously, for a proposed application as the above mentioned of driving a multiphase, typically a three-phase, AC motor, it is necessary to generate three sinusoidal control signals. The control signals are out of phase by 120.degree. among each other, and the frequency and amplitude are regulated by the control system.
Generally, one of the technical problems that is encountered in realizing these systems in a digital form, using a static memory for storing a sequence of a number (m) of digital samples of a sinusoid, each of which is stored in the form of a word composed of a certain number of bits, is due to the way an appropriate scanning frequency of the control sinusoid is established as a function of the frequency of the PWM to be produced. In a digital control system the approach is that of selecting the scanning frequency using a selection word or datum N, producing then the desired scanning frequency thus selected by a block that divides a basic scanning clock frequency by the selection datum N, incremented by one (to avoid an illicit division by zero).
As indicated in FIG. 1, this relationship follows an hyperbolic characteristic. This implies that the resolution becomes more and more difficult for small values of N and this the regulation may become critical. The characteristic depicted in FIG. 1 shows indeed accentuated discontinuities for small values of N. For instance, when setting rather high speeds (corresponding, for example, to a drive frequency of 50 Hz or higher), on the basis of possible binary resolutions in this range (0-25), it becomes difficult to correctly set the desired memory scanning frequency.
Evidently, there exists the need and/or usefulness for an improved method of generating digital streams representing modulated PWM signals that can be implemented using relatively simplified logic circuitry interfaceable to any power driving circuit for a digital DC/AC conversion, and capable of handling the control signals for the power switching devices.